Precipitation manifestations caused by changes in acidification in vegetable tanning extracts are examined and their importance for the assessment of tanning solutions and the tanning process itself are discussed in this article. As manifestations of acid precipitation in vegetable tanning, materials are of great importance. The authors analyzed the activity of hydrochloric and sulfuric acids at varying pH. Commercially available extracts, as well as their own, were used. As the time factor plays an important role, a time limit of l/2 hour was set for precipitation to occur. Rapid coagulation occurs at or near the isoelectric point and is maximal. A slow coagulation takes place when acidification is done by small incremental electrolyte addition. The authors believe that a pH 4.5 to 5 is the most advantageous as the skin pH has a value of 5 to 5.5. They show that tanning should start with extracts of low acid coagulability and finish with easy coagulable solutions. The influence of organic acids (formic, acetic, propionic & lactic) was also examined as well as aging of the substrate. Filtration or other mechanical cleansing of a crude extract has a marked influence on acid coagulability. Cleansing by addition of blood seems not to alter it. Sultitation (by sulfite or bisulfite) has no influence at low concentration but increasing concentrations alters the precipitability and may even transform tanning substances into non-tanning ones.